1. Field of the Invention
The present invention relates to an internal combustion engine, in particular, to an internal combustion engine that has a secondary air passage located above a combustion chamber.
2. Description of the Related Art
Conventionally, an internal combustion engine that has a secondary air passage is known. In such internal combustion engines, an internal combustion engine that has a secondary air passage located above a combustion chamber (see, for example JP-A-2005-248794, JP-A-2000-87739), or an internal combustion engine that has a secondary air passage extending along an exhaust passage (see, for example, JP-A-2000-73752) are known. The secondary air passage introduces secondary air into the exhaust passage, and the introduced secondary air is used for afterburning of the exhaust gas. Afterburning of the exhaust gas reduces unburned components, such as HC and CO, in the exhaust gas. Further, JP-A-2000-73752 describes an internal combustion engine in which a water jacket is formed between a secondary air passage formed along the exhaust gas passage and an upper surface of cylinder heads. This water jacket cools the upper portion of the cylinder heads by coolant, and prevents a packing, a gasket, etc. from being deteriorated by heat.
Meanwhile, as a more specific example of an internal combustion engine that has a secondary air passage located above the combustion chamber, an internal combustion engine is known that has a secondary air passage extending in a direction of cylinder arrangement. Such an internal combustion engine can reduce the number of components or processing sites, as compared to the internal combustion engines described in the three references identified above. In other words, such an internal combustion engine is favorable in cost However, when the afterburning of the exhaust gas is not performed, the high-temperature exhaust gas flows into and stays in the secondary air passage. Accordingly, in such an internal combustion engine, because of its structure, heat radiation from the secondary air passage may cause various adverse affects on the components. More specifically, for example, increase in the intake air temperature reduces volumetric efficiency of the intake air, and the output performance of the internal combustion engine may thus be reduced. Further, when the intake air temperature increases, knocking may occur more easily. Furthermore, for example, the temperature of a valve operating mechanism that operates an intake valve or an exhaust valve may increase, as compared to conventional structures, and reliability may thus decrease. Further, for example, the engine oil staying above the secondary air passage may receive heat, thereby accelerating the deterioration of engine oil by heat.